The Pakihi Bog Began As a Lake When the Surrounding Area Was Forested by Both Podocarps and Silver Beech
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76 PROCEEDINGS OF THE NEW ZEALAND ECOLOGICAL SOCIETY, VOL 22, 1975 A LOWLAND VEGETATION SEQUENCE IN SOUTH WESTLAND: PAKIHI BOG TO MIXED BEECH-PO DO CARP FOREST PART 1: THE PRINCIPAL STRATA A. F. MARK and P. M. F. SMITH Botany Department. University of Otago. Dunedin, New Zealand SUMMARY: Six stands of lowland vegetation representing transitions from pakihi bog to mixed beech-podocarp forest are described quantitatively from the coastal plain of the Arawata and Jackson Rivers in South Westland. The sequence depicts progressive changes in canopy height. biomass and floristic richness. and appears to involve, at least in part. a primary succession. Some boundaries have been sharpene:i by periodic fires but the long+ term trend remains unobscured. Pollen evidence indicates that the pakihi bog began as a lake when the surrounding area was forested by both podocarps and silver beech. The bog seems never to have supported a \'.'oo:l.y vegetation but in the absence of fire. forest would slowly encroach on it. With a highway traversing the sequence. its reservation for educational and scientific pur~ poses has been proposed. INTRODUCTION flat or sloping) type of Paludification (blanket) Bog (Cranwell, 1952). They are characteristically under- A feature of lowland coastal gravel plains and lain either. by peat or by highly leached soils asso- terraces throughout Westland is the occurrence of ciated usually with a loess cap and a strong iron pan openings on poorly drained sites in otherwise ta1l formation which seriously jmpedes drainage. lowland forest (Fig. I). These forest openings or On pakihis not recently disturbed by fire there is pakihis have been classed as a soligenous (surface a well defined sequence of vegetation. The open central part of the bog merges into a woody margin via open scrub which gradually thickens and increases in height from low shrubland through woodland, culminating in tall lowland forest (Fig. 2). The concentric pattern of vegetation types represent- ing this sequence is strongly suggestive of a natural plant succession, as indicated by Cockayne (1928, p. 180). Holloway (1954, p. 381) and others (Chavasse, 1962). Indeed, Holloway (p. 381-2), referring to the patchiness of lowland Westland vegetation, suggests that the pakihi succession is part of a more widespread vegetation response to cJimatic changes occurring some time about the twelfth century A.D. He concedes, however, that the more open pakihis may be stable or at best represent a very slow succession because of recalcitrant soil FIGURE 1. View southeast from the slopes of Mt. conditions. McLean showing the lowland forest and pakihi of In contrast, there is evidence in some pakihis at Dismal Swamp about 2 km northeast of the study least, that they may represent the reverse situation, area. Hindley Creek drains the valley and crosses that is, a forest retrogression associated with soil the pakihi on the far left. May 1965. degradation following a period of apparent steady MARK AND SMITH: SOUTH WESTLAND LOWLAND VEGETATION; PART I 77 The southernmost pakihis may be those on the extensive coastal plain between the Waiatoto and Arawata Rivers which, being south of the Paringa River, are associated with a mixed beech-podocarp forest. This paper describes the typical vegetation sequence in the vicinity of Dismal Swamp, one of the most extensive pakihis on this plain (Fig. 3). The study began as a University of Otago Science Students' project in 1965 (Smith, 1965) but additional information has been collected subsequently. This paper describes quantitatively the principal strata represented in the vegetation sequence. The epiphytic and ground vegetation, in which bryophytes predom- inate, is described separately as Part 2 (Scott and Rowley. 1975). FIGURE 2. Vegetation sequence along pakihi edge in the study area at Dismal Swamp. Open short manuka THE STUDY AREA scrub in the foreground grades into increasingly Physiography tall dense manuka woodland, followed by silver pine (dark rounded canopies) with young podocarp forest, The area studied is underlain by gravel, sand or especia'ly rimu, silhouetted on the skyline. May silt (Mutch and McKellar, 1964). It is situated about 1965. 4 km inland and ]2 m above sea level. Since the highway immediately north of the Arawata River bridge traverses the entire vegetation sequence, sites state characterised by tall lowland forest. This for sampling were selected adjacent to it (Fig. 3). process, through developing a highly infertile gley The highway not only facilitated access to this low- podzol with associated iron pan, results in soil con- lying and featureless country but also ensured ditions inhibitory to the lowland forest and allows minimal interference by deer. the establishment of silver pine (Dacrydiul11 A small isolated outcrop of undifferentiated colensoi)* (Stevens, 1968). Rigg (1962) even claims metamorphic rock which the highway crosses about that pakihis occupying glacial and interglacial I km north of the bridge (Mutch and McKeIJar, terraces in the vicinity of Westport, north Westland, 1964) has somewhat higher relief and was avoided. have developed largely from forest due either to soil deterioration under forest or to climatic change, Climale or both. Further, she suggests the possibility of Official Meteorological Service records for two several cycles of succession and retrogression since nearby coastal stations, Jackson Bay] I km to the most of the larger pakihis contain buried timbers. north-west and Haast, located 29 km northwards However, she admits that the successional phase along the coast, indicate an annual rainfall somewhat is now widespread and has been "for many in excess of 3,500 mm fairly evenly spread over centuries", although recent burning together with about 200 days, and a mean annual air temperature increased waterlogging due to reduced water con- of about 11°C with relatively slight diurnal and sumption following logging, have counteracted this seasonal fluctuations. Screen frosts occur on 3 to 9 tendency at least in the Westport area. This latter days between May and October while ground frosts claim, however, finds no support from McDonald's may occur on 53 to 70 days throughout the year. Fog (1955) comparison of soil moisture content and and snow are both uncommon (12 year means of 8.5 other physical properties between forest and adjacent and 0.5 days, respectively, for Jackson Bay). sites reverting to pakihi following logging operations. Despite the large number of rain days, sunshine duration (for Haast) is relatively high-I,816 hours * Nomenclature follows Allan (1961) for pteridophytes, annually or 43 % of the possible total. gymnosperms and dicotyledons, Moore and Edgar (1970) for monocotyledons except for the grasses which follow Cheeseman (1925) or where applicable, METHODS Zotov (1963). Otherwise authorities are given in the species list (Appendix, Table I). Voucher specimens of Six sites were chosen to represent the vegetation most species listed are deposited in the OT A sequence from bog near the centre of the pakihi to herbarium. fully developed lowland forest close to the Arawata 78 PROCEEDINGS OF THE NEW ZEALAND ECOLOGICAL SOCIETY, VOL. 22, 1975 LOCA rJON '..' " .j"" ' Stond location "'" Moture lowland.jor'est (hilly) Malure lowland forest (flatland) Young rimu forest /' Silver pine wOOdland Monuko Shrublancl '09 FIGURE 3. The coastal plain between the Wala/olo and Arawata Rivers, South Westland, showing distribution of the main vegetation types and the approximate location of six stands studied to describe the vegetation sequence. The flatland east of the highway is generally known a.v Dismal Swamp, that to the west as the Burmeister Morasse. The map was compiled from an enlargement of aerial photograph 3977/11 supplied by N.2. Aerial Mapping Ltd., Hastings, and with a.fsistance of an unpublished National Forest Survey forest type map supplied by the New Zealand Forest Service. River (Fig. 3). For all except the herbaceous plant (iii) Shrubs: woody species 0.3 to 5 m tall (composi- cover on the pakihi the point.centred quarter method tion, density); of plotless sampling (Cottam and Curtis, 1956) was (iv) Subshrubs: woody species < 0.3 m tall (compo- used, as for previous studies involving a similar range sition, density); of vegetation (Mark, et at., 1964). (v) Herbs: herbaceous species> 15 em tall (com- The categories (and values recorded for each) were: position, density). , (i) Trees: woody species> 10 cm d.b.h. (composi- Fifty points were used in all except the mature tion, density, basal area);' forest stand where the number was doubled for the (ii) Small-trees: woody species < 10 cm d.b.h. but> small-tree, shrub and suhshrub layers to cope with 5 m tall (composition, density); their increased floristic diversity. Points were distri- MARK AND SMITH: SOUTH WESTLAND LOWLAND VEGETATION; PART 1 79 STAGE QRACOPHYlLUM LONGIFOLJUM LEPTOSPERMUM SCoPAR1UM QACRYOJUM COLEN SO! PHYLLoCLAOUS ALPINUS CoPROSMA FoETJDlSSJMA COPROSMA COLENSO! NEOMYRTUS PEOUNCULATA NEoPANAX COLENSO! MYRSINE DIVARICATA DACRYDIUM CUPRESSJNUM PODDCARPUS HALLI! NOTHOFAGUS SOL v CLIFF DACRYCARPUS DACRYDJOIOES PSEUOOPANAX CRASS!FOLJUS WEINMANN!A RACEMOSA NOTHOFAGUS MENZIES!! ELAEOCARPUS HoOKERIANUS DlCKSONIA SOUARROSA POOOCARPUS FERRUGINEU5 PSEUDOWINTERA COLORATA CANOPY HEIGHT 1m) ) , 0.' " " " Subshrubs Novalu. Novalu. 3~.SOO 3QOO 7,100 TOTAL 85.000 Shrubs 6,500 53,~00 13,900 22.900 31.~00 6.000 DENSITY Small"trus - - 5..70 ..0'30 2.800 1.030 (sttms/ha,) { Trus - 1,130 1,OeO '" TOTAL BASAL AREA -T,us - - I, "~9 ~5S9 6.551 (dm'ilha1 FIGURE 4. Values for relative density of the important woody species (>5% in one or more stands) in four size claj'ses (see key) from six stand... of vegetation (StaRes ]-6) at Dismal Swamp, South Westland. Values entered for subshl'ubj. in StaRes] and 2 are re/ative.J::Qver values. Relative basal area of tree species is also included (open blocks) while canopy heights and stand totals for density and basal area are shown at the bottom.